Effects of Shading the First Leaf on Growth of Barley Plants: II. Effects on Photosynthesis

The contributions made by photosynthesis in the first leaf to seedling growth have been examined by a variety of methods including infra-red gas analysis and the use of 14CO2. The first leaf is fully expanded by day 8 and maximal rates of photosynthesis are achieved about I day later. Up to day 8 growth of the seedlings results from the redistribution of seed reserves and once these are exhausted growth is dependent upon the first leaf, being reduced to very low levels if this is shaded. The second leaf which begins to expand rapidly after day 10 is contributing to growth by day 14, and the contribution from the first leaf begins to decline after day 12. Apart from greatly reducing photosynthesis in treated leaves, shade also affects the development of photosynthetic capacity. When applied for 2 days or more from day 6, shade reduces the peak level of carbon fixation achieved on days 9 and 10 by up to 35 per cent. It is shown that development of the first leaf in terms of increased dry weight and photosynthetic capacity, is dependent on photosynthesis in the developing leaf itself. The mechanisms by which shading affects development are discussed.